1. Field of the Invention
The present invention relates to a signal generation circuit for controlling the rotation of a motor such as a compressor or stepping motor, for example, in an air-conditioning system possessing an inverter circuit, using a pulse width modulation (PWM) signal.
2. Description of the prior Art The voltage variable method, by transformer tap changeover or phase control, has been used for controlling the rotation of a stepping motor or three-phase alternating current motor, for example, in an air-conditioner. However, because of the problems of shortage of motor torque or generation of noise, the inverter method has been recently employed. In this method, a signal generation circuit is used consisting of one-chip microcomputer or the like, and the control signal is delivered from the signal generation circuit to control the direct-current/alternating-current inverter, and the rotation is controlled smoothly by driving the motor with a frequency corresponding to a desired rotating speed.
FIG. 1 is a block diagram showing the hardware for controlling a compressor 12 using a signal generation circuit 1 of prior art. The signal generation circuit 1 comprises a register 4 connected by way of a data bus 2, a read-only memory (ROM) 5, a central processing unit (CPU) 3, an intenal timer 6, and a register 7 for input and output.
Operation program and waveform data of the signal generation circuit 1 are stored in the ROM 5. The CPU 3, according to the operation program, refers to the data stored in the register 4, and selectively reads out the waveform data from the ROM 5. Furthermore, on the basis of the data stored in the register 4, data conversion and other signal processing is done, and the signal is delivered to outside through the register 7 for input and output. At this time, an interruption signal generated by the internal timer 6 is given to the CPU 3 through a line 8, and the timing of the output signal is controlled by it.
The signal delivered from the signal generation circuit 1 is given to transistors T1 to T6 of an inverter 13 through a delay circuit 9 and a photocoupler 10. The delay circuit 9 is installed in consideration of the turn-off time in the transistors T1 to T6, and the photocoupler 10 is intended to prevent the incoming noise from entering the signal generation circuit 1. The transistors T1, T2, the transistors T3, T4, and the transistors T5, T6 are respectively connected in series, and supply voltages from a power supply circuit 11 are connected parallel to the collectors of the transistors T1, T3, T5. Output signal S1, S1 of the photocoupler 10 are respectively applied to the bases of the transistors T1, T2, output signals S2, S2 are applied to the bases of the transistors T3, T4, and output signals S3, S3 are applied to the bases of the transistors T5, T6. Signals at the connection junction of transistors T1, T2, at the connection junction of transistors T3, T4, and at the connection junction of transistors T5, T6 are given to the compressor 12 through lines la, lb, lc, respectively.
The compressor 12 is operated, for example, by a three-phase alternating-current motor, and may be equivalently expressed by coils 12a, 12b, 12c. The ends at one side of coils 12a to 12c are mutually connected, and the lines la to lc are connected to the other ends of the coils 12a to 12c, respectively.
Thus, the direct-current output from the power supply circuit 11 is converted into a rectangular wave equivalent to three-phase alternating-current output by the inverter 13, and is applied to the coils 12a to 12c, so that the operating state in the compressor 12, that is, the rotating speed of the motor is controlled.
When controlling the operating state of the compressor 12 by using the signal generation circuit 1, it is necessary to prepare a program to be stored in the ROM 5 in order to operate the signal generation circuit 1. According to the program, all actions including reading of waveform data from the ROM 5 and conversion of waveform data are conducted, and therefore the program itself becomes huge, which may increase the load when writing software. Still more, the waveform of the output signal changes very slightly depending on the time for executing the subroutines of the program or the time for executing the commands included in the program (cycle time). Therefore, this signal generation circuit 1 cannot generate a signal possessing a pulse waveform of high precision.